Russia has denied foreign Greenpeace activists permission to leave the country pending their trial over an Arctic protest, the group said on Friday, some two weeks after they were released on bail.

Dutch activist Anne Mie Jensen received a letter from the Investigative Committee, in which the powerful agency indicated to the crew, indicted on hooliganism charges, that they "cannot leave the country," Greenpeace said.

The organisation said they had asked the committee to request exit visas from the Russian migration services for the 26 foreign nationals, so that they can go home, but the committee has refused.

The activists "would now be forced to stay in Saint Petersburg for Christmas and possibly well beyond," without any idea as to when authorities will actually summon them for questioning.

The 30-strong crew of Arctic Sunrise was arrested after some activists attempted to scale a Gazprom gas company platform in the northern Barents Sea to protest oil exploration in the vulnerable area.

Russian special forces boarded the ship shortly after the protest and towed it into Murmansk.

After spending almost two months in the Murmansk jail on charges of piracy and hooliganism, the group, which included journalists, was moved to Saint Petersburg and suddenly released on bail last month. Currently only the hooliganism charge remains in the probe.

Late last month a senior Kremlin official said that activists were free to leave Russia. "Nobody will hold them," chief of staff Sergei Ivanov said.

Migration officials have told Greenpeace lawyers that they require authorisation from investigators to grant exit visas to the foreigners, who hail from 17 different countries.

But in their letter the committee said the foreigners could not be granted exit visas because their passports do not have Russian entry stamps, spokeswoman Maria Favorskaya told AFP.

Favorskaya said the Investigative Committee's decision was "unfounded," adding that according to a court ruling the activists were free to travel.

The Greenpeace affair has sparked criticism in the West and prompted celebrities such as Madonna and Paul McCartney to appeal for the crew's freedom.

The ship Arctic Sunrise is still detained at port in the Russian northern city of Murmansk, despite a ruling last month by the International Tribunal for the Law of the Sea that the ship be released, and a 3.6 million euro bond posted for the vessel by the Netherlands.